Heterojunction structures such as enhancement/depletion mode structures which employ only N type FETs and complementary HFET structures which comprise a P type FET and an N type FET are well known in the semiconductor art.
Complementary HFETs are most often employed for high speed, low power applications. These applications require extremely low gate leakages. However, the types of complementary HFET structures well known in the art often have relatively high gate leakages which cause detrimental effects. One reason for the relatively high gate leakages in these structures is that the gate barrier heights of the devices are too low. Accordingly, it would be desirable to increase the gate barrier heights of the devices to reduce gate leakage.
High speed, low power applications also require that the threshold voltages of devices be relatively low. Delta doping techniques are known in the art and are used to adjust threshold voltage. One structure that employs N type delta doping includes gate metal being formed directly on an insulator and a thin (atomic scale) N type barrier layer which is highly doped on the order of 4 .times.10.sup.18 atoms/cm.sup.3. The threshold voltage may be changed by varying the dopant concentration of the barrier layer. However, the barrier layer cannot be doped at a concentration necessary to enable both the N FET and the P FET of a complementary HFET structure to have sufficiently low threshold voltages. Because the N FET and P FET are coupled in a complementary HFET structure, the threshold voltage will decrease for the N FET while increasing the P FET and vice versa for P type delta doping. Accordingly, it would be desirable to have a complementary HFET structure wherein the threshold voltage of both the P FET and the N FET may be kept relatively low.